The goal of this project is to investigate the dynamical effects of short-period, small-scale gravity waves as modulated by the dynamic and horizontally inhomogeneous structure of the terrestrial mesosphere and lower thermosphere (MLT) region. Although such waves contribute significantly to the structure and variability of the MLT, the present understanding of gravity wave generation, propagation, and dissipation has not taken the effects of local structure into account and thus may suffer from observational bias. Specific science questions related to multi-scale gravity wave interaction will be addressed in the research work through both 3-D numerical modeling, using computing cluster hardware acquired as part of this project, as well as observations of the MLT by airglow imagers, radars, and lidars at multiple facilities. The data will be used to constrain the model simulations as well as validate its results. The project will also provide a theoretical foundation for future tomographic imaging studies of wave induced airglow and temperature perturbations in order to facilitate more accurate assessments of gravity wave fluxes and multi-scale structure.
Much like the ocean, the Earthâ€™s atmosphere supports a broad spectrum of waves. Of particular interest to this project were atmospheric gravity waves, which are buoyant wave motions that are readily generated by weather systems. They have periods of several to tens of minutes, and wavelengths of tens to hundreds of kilometers, and can propagate upward to altitudes often exceeding 100 km. Gravity waves are important for their role in carrying momentum and energy from the lower atmosphere into the upper atmosphere. As the waves deposit their energy and momentum, they influence the flows and thermal structure of the upper atmosphere. This is a primary dynamical coupling process between atmospheric regions, and clear understanding of the effects of these waves is required to model and predict the structure of the upper atmosphere. This project was motivated by the fact that propagation of these waves, and their deposition of momentum and energy, is strongly influenced by the structure of the atmosphere including the variations of winds and temperatures associated with other waves and flows. The evolutions of waves under variable atmospheric conditions were investigated using computational models. Case studies were designed to address science questions on the effects of the atmosphereâ€™s structure and dynamics on the propagation and dissipation of the waves and, likewise, on the effects of the waves on the atmosphereâ€™s structure and dynamics. Computational models and observational data, especially when used together, can elucidate the roles that these waves have in the dynamics of Earthâ€™s upper atmosphere. This project revealed several important features of middle- and upper-atmospheric gravity wave propagation, which were also supported by observational evidence. One finding was that a very large fraction of waves observed at altitudes of 80-100 km are significantly influenced by the structure of the atmosphere, to the extent that they are likely to experience reflection at some altitudes, which may prolong their propagation. This is the case even where atmospheric temperature variations, rather than winds, can play a dominant role in the waves' propagation. Another finding is that waves may at times become trapped by reflection at lower regions of the atmosphere, to gradually "leak" upwards at later times and longer distances. This again tends to prolong the propagation of the waves, and to distribute their effects over longer distances. The results of these studies, conducted by the principal investigator in collaboration with undergraduate and graduate students, were ultimately published in several papers and a thesis, and presented at workshops and conferences. The computational models developed and enhanced as part of the project will be well-utilized into the future. As additional broader impacts, the project provided important development opportunities for the students, thus enriching their academic experiences, by involving them directly in scientific research. It also provided the foundation for development of a new graduate-level course in computational atmospheric dynamics.